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Nickel and other super alloys

EF420 Lecture 10

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Nickel

The name ‘nickel’ is from the German for ‘Old Nick’s (the Devil’s) copper’ a derogatory term for the hard useless metal yielded by what was thought to be a copper-bearing ore

These days nickel is an important alloy element, with much wider application than copper

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Unusual properties

Nickel-iron with low expansion coefficient (Invar) Nickel-chromium resistance heating elements Shape memory alloys (Ni-Ti) Alloys with soft magnetic properties Superalloys for gas turbine engines and other

high temperature uses

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Nickel

fcc metal, no solid phase changes. Grey colour 43% of production used in stainless steels 20% in non-ferrous alloys Alloy steels, foundry products Other uses

Chemicals, catalysts, ceramics, coinage, magnets

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Nickel alloys

About 60 alloys Some with less than 50% nickel (or any other alloy

element) Single phase alloys (fcc)

Nickel has a high solubility of Cu, Cr, Mo, Fe, etc

Precipitation hardened alloys With Al, Ti or Nb in above 0.5%

Dispersion strengthened alloys Produced by powder metallurgy

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Alloy designations

The alloy names are based upon proprietary names, usually from Inco, Haynes, Krupp-VDM,

eg Alloy 625 was originally Inconel® 625

UNS number is a 5-digit numbers preceded by ‘N’ eg Alloy 625 is UNS N06625

ISO 9722 numbers are often (but not always) based upon the UNS numbers

eg UNS N06625 is ISO NW6625 DIN designations are descriptive (NiCr22Mo9Nb)

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Properties

Corrosion resistance to neutral, acid, basic, oxidising or reducing environments

Piping, vessels for chemicals, seawater, etc

High temperature strength, corrosion and oxidation resistance

Up to 1200˚C

High cost Nickel alloy weld overlays commonly used

Nickel alloys are excellent filler metals for dissimilar welds

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Metallurgy of nickel alloys

Alloys are mostly single phase fcc Nickel has a wide range of solubility for other metals Alloys contain Cu, Cr, Mo, Fe,

Age hardenable alloys contain aluminium, niobium and/or titanium

Dispersion-strengthened alloys contain 2% thoria (ThO2) and are made by powder metallurgy.

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Commercially pure nickel

Nickel 200 Up to 0.15% C as impurity; graphitises at over 320˚C Resists caustic soda & other alkalies

Nickel 201: 0.02% C maximum Duranickel 301: Ni-4.4Al-0.6Ti

Age hardening gamma prime

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Applications of pure nickel

Caustic soda handling equipmentFood processing

Laboratory crucibles Chemical shipping drums Electrical and electronics parts

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Nickel-copper (Monel metal)

Probably the best corrosion resistance to a wide range of environments of any alloy

Monel® 400: 70Ni-30Cu fcc alloy: Rm 480 to 1170 MPa Cladding, vessels and piping for seawater, brackish water,

chlorinated solvents, many acids and alkalies

Monel 405: 0.04% S - free machining Monel K-500: 2.7% Al, 0.6% Ti: Rm 1100 to 1240 MPa

Age hardening - gamma prime phase

Steam turbine blades, etc

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Nickel-chromium alloys

Over 50% Ni with >15% Cr, Mo, Fe, C, W Single phase fcc, solid solution strengthened

Seamless transition to austenitic stainless steel

Wide range of temperatures (cryogenic to 1000˚C)

Acids, neutral and alkalies. Oxidising and reducing chemicals.

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Examples of Ni-Cr alloys

Nichrome alloys 80Ni-30Cr, 60Ni-16Cr-24Fe Electrical resistors & heating elements

Inconel® 600, 625, 671, 690 Eg Inconel 625: 61Ni-21Cr-9Mo-3.6Nb

Hastelloy® C-4, C-276, G, G-3, N, S, W, X Eg Hastelloy® C-276: 59Ni-15.5Cr-16Mo-3.75W-5.5Fe

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Nickel-iron-chromium alloys

Cross between austenitic stainless steel and nickel alloys

Incoloy® 800: 32.5Ni-21Cr-46Fe RA333: 45Ni-25Cr-18Fe-3Mo-3Co-3W

Excellent resistance to oxidation at high temperatures, Good resistance to corrosion in acids and salts, but not

halides

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Fabrication of Ni alloys

Casting is difficult, but castings are produced Forming (hot & cold) is similar, but somewhat more

difficult than austenitic stainless steels Weldability is excellent for many types (better than

austenitic stainless steel) Heating operations are generally performed in controlled

atmospheres to avoid intercrystalline embrittlement Avoid contamination with S, P, Pb, Zn, Sn

Solidification or liquation cracking

Welding procedures

GTAW, GMAW, MMAW, SAW, PAW, OFW, RW, EBW, Use matching or over-alloyed fillers Keep arc energy low

Segregation can result in loss of corrosion performance

Preheat not required Keep clean to avoid cracking Remove slag to avoid corrosion attack at high

temperatures

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Super alloys (high temperature)

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Super alloys

High temperature performance (strength) Gas turbines, steam turbines, reciprocating

engines Hot working and casting tools and dies Aircraft & space vehicles Heat treatment trays, fixtures, conveyors Nuclear and chemical industries

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Alloy types

Selection depends on strength, creep & oxidation performance at the elevated temperature

Iron based alloys Nickel based alloys Cobalt based alloys Refractory metals (niobium, molybdenum, tantalum,

rhenium & tungsten) Directionally solidified eutectics, single crystals,

intermetallic compounds Non-metals (graphite, ceramics)

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Fabrication

Fabrication & machining of many super alloys is difficult

No hot or cold forming Investment casting Powder metallurgy Electrochemical, electrodischarge or ultrasonic

machining

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Metallurgy of Fe, Ni & Co super alloys

Solid solution strengthened Precipitation hardened Oxide-dispersion strengthened

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Cobalt-based alloys

Resistant to specific environments, eg engine combustion gases at high temperature

High strength Ry up to 790 MPa & Rm up to 1170 MPa at room

temperature 1000 hour rupture strength at 815˚C up to 230 MPa

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Typical cobalt-base alloys

650˚C to 1150˚C Haynes® 25, 188

Fasteners at 650˚C UMCo-50, S-816

Wear alloys Stellite® 6B

fcc with some tendency to transform to cph Carbides and intermetallic compounds

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Nickel-based super alloys

Ni alloys have a better resistance to high temperatures and have a higher strength than stainless steels or cobalt alloys

Ry up to 1200 MPa & Rm up to 1450 MPa at room temp 1000 hour rupture strength at 850˚C up to 450 MPa

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Solid solution alloys

Inconels, Hastelloys, RA-333 used for furnace parts

Nimonic® 75 for gas turbines

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Precipitation hardened alloys

Aluminium and titanium, gamma prime or gamma double prime precipitates

Inconel X-750, Nimonic 80, Aerospace applications

Gas turbine blades

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Welding the PH Ni alloys

GTAW process only To avoid burn-out of strengthening alloys in fillers

Cracking can be experienced Anneal base material before welding Stress relieve and age weldments This also ensures welds have highest strength

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References

Nickel development institute (NiDI) provides much free literature on nickel alloys and stainless steel

Nidi.org Haynesintl.com ASM Handbook Vol 3